Reconfigurable DNA nano-tweezer

ABSTRACT

A photocaged DNA nano-tweezer and methods of using said photocaged DNA nano-tweezer are described. In particular, provided herein is a DNA nano-tweezer comprising a hairpin with a single-stranded loop that comprises a first arm and a second arm; and a trigger strand complementary to the single-stranded loop and comprising at least one photocaged residue with a protecting group.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/746,139, filed Oct. 16, 2018, which is incorporated herein in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under FA9550-17-1-0053awarded by the Air Force Office of Scientific Research (AFOSR). Thegovernment has certain rights in the invention.

REFERENCE TO A SEQUENCE LISTING SUBMITTED VIA EFS-WEB

The content of the ASCII text file of the sequence listing named“112624_01126_ST25.txt” which is 10.4 kb in size was created on Oct. 14,2019 and electronically submitted via EFS-Web herewith the applicationis incorporated herein by reference in its entirety.

BACKGROUND

One of the central goals of nanotechnology is to build machines,switches, or reconfigurable devices at the nanoscale level that canoperate in a stimulus-responsive manner. These constructs would haveapplications in studying receptor-ligand interactions (Wang et al.2013), releasing cargo for drug delivery (Douglas et al. 2012), orconstructing dynamic materials such as artificial muscles (Bruns et al.2014). DNA is highly promising molecular building block for creatingsuch systems owing to the exquisite programmability of Watson-Crickpairing, allowing for simple assemblies based on just a few strands (Loet al. 2010, Rothemund et al. 2004, Winfree et al. 1998, Yan et al.2003, Zheng et al. 2009) or highly complex and anisotropic structuresusing techniques like DNA origami (Douglas et al. 2009, Hong et al.2017, Rothemund et al. 2006) or 3D bricks (Ke et al. 2012, Wei et al.2012). In recent years, a whole suite of nanoscale analogues ofmacroscopic mechanical elements and devices have been reported,including hinges/calipers (Funke et al. 2016), pistons (Marras et al.2015) boxes with addressable latches (Douglas et al. 2012, Jiang et al.2018) or interlocked rotaxane nanostructures (List et al. 2016, Powellet al. 2016). By far the most common way to actuate these constructs isthrough the addition of single-stranded nucleotides that can reconfigurethe structure through toehold-mediated strand displacement (Zhang et al.2011) whereby an oligonucleotide outcompetes a shorter strand in orderto break and replace a DNA hybridization interaction. Despite theability to programmably and orthogonally control multiple elementsthrough specific trigger strands, this approach has the disadvantagethat the strand must be added externally, limiting its use inapplications such as inside of cells, or in vivo, and often in highmolar excess to achieve suitable kinetics.

SUMMARY

In a first aspect, described herein is a DNA nano-tweezer comprising ahairpin with a single-stranded loop that comprises a first arm and asecond arm and a trigger strand complementary to the single-strandedloop and comprising at least one photocaged residue with a protectinggroup. In some embodiments, the single-stranded loop is a poly-A loopthe trigger strand is a poly-T strand. In some embodiments, thesingle-stranded loop is a poly-C loop and the trigger strand is a poly-Gloop. In some embodiments, the single-stranded loop and the triggerstrand are selected from the group consisting of a poly-A loop, a poly-Tloop, a poly-G loop, and a poly-C loop.

In some embodiments, the protecting group is a 6-nitropiperonyloxymethylprotecting group. In some embodiments, the DNA nano-tweezer additionallycomprises a locking strand. In some embodiments, the locking strandcomprises an o-nitrobenzyl ester photocleavable backbone.

In some embodiments, the DNA nano-tweezer additionally comprising atleast one fluorescent label. In some embodiments, the DNA nano-tweezeradditionally comprising a ligand.

In some embodiments, the distance between the first arm and the secondarm is between about 1 nm and about 10 nm. The distance between thefirst arm and the second is measured between the distal end of the firstarm away from the hinge and the distal end of the second arm away fromthe hinge, as demonstrated in FIG. 1A.

In a second aspect, provided herein is a method of inducing aconformational change in nanostructured DNA, the method comprising thestep of exposing a DNA nano-tweezer as described herein to a pulse oflight, whereby the DNA nano-tweezer undergoes a conformational changefrom a closed conformation to an open conformation. In some embodiments,the protecting group is a 6-nitropiperonyloxymethyl protecting group andthe light is UV light.

In some embodiments, the light have a wavelength between about 300 nmand about 400 nm. In some embodiments, the pulse of light is betweenabout 1 second and about 10 seconds.

In a third aspect, described herein is a DNA nano-tweezer comprising ahairpin with a single-stranded loop, wherein the loop has at least twoarms with a distance of between about 4 nm to about 18 nm between the atleast two arms, and a trigger strand complementary to thesingle-stranded loop and comprising at least one photocaged residue,wherein the DNA nano-tweezer is in a closed conformation until exposedto a pulse of light whereby the photocaged residue is released and thetrigger strand is hybridized to the single-stranded loop forming an openconformation wherein the distance between the at least two arms is atleast 18 nm. In some embodiments, the single-stranded loop and thetrigger strand are selected from the group consisting of a poly-A loop,a poly-T loop, a poly-G loop, and a poly-C loop.

In some embodiments, the photocaged residue comprises a6-nitropiperonyloxymethyl protecting group. In some embodiments, the DNAnano-tweezer additionally comprising a ligand. In some embodiments, theDNA nano-tweezer additionally comprising a fluorescent label.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, and patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF DRAWINGS

The patent or patent application file contains at least one drawing incolor. Copies of this patent or patent application publication withcolor drawings will be provided by the Office upon request and paymentof the necessary fee.

FIGS. 1A-1E show the design of photocaged tweezers and characterizationof photocaged strands. A) Design and dimensions of original tweezer,which requires an external trigger strand (blue) to open hairpin loop(red). B) Design of photocaged tweezer with internally incorporatedtrigger strand (blue). (hv—high frequency light) C) UV/Vis spectra ofphotocage deprotection. D) Denaturing PAGE gel characterization ofphotocage deprotection: lane M, ssDNA ladder (nt); lane 1, non-protectedcontrol strand; lane 2, photocaged strand; lane 3, photocaged strandafter deprotection. E) Native PAGE gel characterization of photocagedeprotection: lane M, dsDNA ladder (bp); lane 1, non-protected controlstrand; lane 2, control strand bound with a complementary polyA strand;lane 3, mixture of photocaged trigger strand and poly(A) mimic ofhairpin strand; lane 4, sample in lane 3 after deprotection.

FIGS. 2A-2C show characterization of dynamic tweezer actuation withlight. A) Distance histograms between the ends of the tweezer arms,based on AFM imaging. Systems: i) photocaged tweezer, ii) originaltweezer actuated by an external trigger strand, iii) photocaged tweezerwith photocleavable locking strands, iv) negative control for photocagedtweezer without locking strands, v) negative control for photocagedtweezer with locking strands. Scale bars: 15 nm. B) Detailed design ofphotocaged tweezer System (iii) with locking strands. C) FRET efficiencymeasurements for Systems (i)-(v). Error bars indicate the standarddeviation for n=3 independent measurements. Sequences included in FIGS.2A-2C include A₁₃ (SEQ ID NO: 1), (XT)₆X (wherein X is a photocagedthymidine residue, SEQ ID NO:2), TAAGACCCACAAT (SEQ ID NO:3),ATTGTGGGTCTTA (SEQ ID NO:4), and T₁₃ (SEQ ID NO:5).

FIGS. 3A-3B show normalized kinetic curves of the tweezer opening. A)Time-dependent FRET of the photo-actuated tweezer at 10 nm and 60 nm; noconcentration dependence was observed. The rate constant was acquired byfitting the kinetic curve to an integrated first order reactionequation. B) Time-dependent FRET of the externally actuated tweezer at60 nm, with varying concentrations of the trigger strand. The rateconstant was acquired by fitting the kinetic curve to an integratedsecond order reaction equation. Insets: zoom-in plot of the first 50 sof the corresponding kinetic curves.

FIGS. 4A-4D show DNA sequence design for original DNA nano-tweezers withrandom loop sequence, polyA loop sequence and photocaged DNAnano-tweezers. A) The original DNA nano-tweezers with random loopsequence (SEQ ID NOs:13, 15, 17, 18, 19, 20, 21, 22, and 23, FIG. 2(ii)). Opening of hairpin depends on addition of external triggerstrand, which is complementary to the hairpin. B) The original DNAnano-tweezers with polyA loop sequence (SEQ ID NOs:13, 15, 17, 18, 19,20, 22, 23, and 34, FIGS. 8A-8B). C) Photocaged DNA nano-tweezerswithout locking strands (SEQ ID NOs:13, 15, 17, 18, 19, 20, 22, 23, 24,and 25, FIG. 2(i)). D) Photocaged DNA nano-tweezers with locking strands(SEQ ID NOs: 13, 15, 17, 18, 19, 20, 24, 25, 26, and 27, FIG. 2 (iii)).

FIGS. 5A-5C show DNA sequence design and gel characterization forinitially designed nano-tweezer with 4 caged residues and UV-Vis spectraof homemade photocaged DNA oligos A) Design of a nano-tweezer withrandom loop sequence (red) and 4 caged dT residues (X, blue) in theinternal trigger strand (blue) using NPOM (SEQ ID NOs:13, 14, 16, 18,19, 20, 22, 23, 32, and 33). B) Native PAGE gel characterization ofphotocaged tweezer deprotection. Lane M: ds DNA ladder (bp); lane 1: Theoriginal closed DNA nano-tweezers with random loop sequence (FIG. 2A(ii)); lane 2: The original opened DNA nano-tweezers with random loopsequence; lane 3: positive control of 4-caged nano-tweezer without NPOMresidues; lane 4: photo-tweezers with 4 cages before UV deprotection;lane 5: photo-tweezers with 4 cages after UV deprotection; lane 6:re-annealed photo-tweezers after UV deprotection. C) UV-Vis spectra of 3homemade photocaged DNA oligos (S1: XGXACAGTTACCGTGTGGTTGCATAGGXAXAC(SEQ ID NO:6); S2: TACCGTGTGGTTGCTGXXGXC (SEQ ID NO:7); S3:CAGACGACCATCTGGACAGAAAAAAAA XXX CXGXCC (SEQ ID NO:8); in each of S1, S2,and S3, “X” represents a photocaged thymidine residue). C1 (SEQ IDNO:35), C2 (SEQ ID NO:36), and C3 (SEQ ID NO:37) are DNA controls, ofS1, S2, and S3 respectively, wherein the thymidine residues are uncagedand represent the strands produced after the exposure of the caged S1,S2, and S3 strands to UV light.

FIG. 6 is an image of a representative UV experiment setup. Allexperiments except cell viability assay were performed with DymaxBlueWave® 200 version 1.1 UV light-curing spot lamp system (an olderversion). Maximum light intensity is ˜18.2 W. The end of optical fiberwas placed ˜1-2 cm above sample surface and a timer for 3 s exposuretime was set prior to measurement.

FIG. 7 shows FRET measurements of nano-tweezer samples Systems (i) to(ix). (i) photocaged tweezer, unlocked; (ii) original tweezer actuatedby an external trigger strand; (iii) photocaged tweezer with lockingstrands; (iv) analogue of System (i) with polyT₁₃ loop and photocagedinternal strand; (v) analogue of System (iii); (vi) control tweezer withpolyA loop and internal polyT strand; (vii) locked tweezer with polyAloop and internal polyT strand; (viii) control tweezer with onlyinternal photocaged strands; (ix) locked tweezer with only internalphotocaged strand. Lane 1 and black curve: tweezers with Cy3 only beforeUV exposure; lane 2 and blue curve: tweezers with Cy3 only after UVexposure; lane 3 and red curve, tweezers with Cy3-Cy5 before UVexposure; lane 4 and purple curve: tweezers with Cy3-Cy5 after UVexposure.

FIG. 8A-8D show FRET measurement of a nano-tweezer (FIG. 1B) andemission spectra of Cy3/Cy5 labeled DNA. A) Bulk FRET measurement ofnano-tweezer in FIG. 1B. B) Real-time Cy5 emission of nano-tweezer inFIG. 1B. C) Emission spectra of Cy3 labeled DNA oligo before and afterUV exposure. D) Emission spectra of Cy5 labeled DNA oligo before andafter UV exposure to confirm that the fluorophores were notsignificantly damaged by the UV illumination.

FIG. 9 shows Zoom-out AFM images of System i. Scale bar: 100 nm.

FIG. 10 shows Zoom-out AFM images of System ii. Scale bar: 100 nm.

FIG. 11 shows Zoom-out AFM images of System iii. Scale bar: 100 nm.

FIG. 12 shows Zoom-out AFM images of System iv. Scale bar: 100 nm.

FIG. 13 shows Zoom-out AFM images of System v. Scale bar: 100 nm.

FIGS. 14A-14D shows an exemplary configuration of a simulated systemfor: (a) unbound strands, (b) fully formed duplex without a kink, withthe choice of stiffness k=11.4 pN/nm, (c) kinked configuration of aformed duplex (with a broken base pairing interaction indicated by anarrow) for a choice of k=2240 pN/nm, (d) kinked configuration with abroken stacking interaction (indicated by an arrow) between subsequentbase pairs (for k=48.63 pN/nm). The two nucleotides between which thespring potential V acts are connected by a schematic image of a spring.

FIG. 15 shows mean distance between the bases that are bound by a springpotential as a function of the number of base pairs formed between thetwo complementary sequences in the simulation. As spring stiffness kgets larger, the distance gets closer to r₀=1 nm, the distance at whichthe spring potential V reaches its minimum energy.

FIG. 16 shows mean energy of the spring potential V (defined in Eq. 3)as a function of the number of base pairs formed between the twocomplementary strands for different values of spring stiffness k. As kgets larger, the duplex prefers to kink rather than stretch the springand hence the mean energy of the spring potential is lower for highervalues of k.

FIG. 17 shows mean force exerted on the spring connecting the two basepairs at the end of a duplex as a function of number of base pairsformed between the two complementary strands.

FIG. 18 shows free energy of the (partially) hybridized duplex of thetwo complementary strands. The probability of the system adopting agiven state is proportional to exp(F/kBT), where F is the free energy ofthe given state. Note that as the spring stiffness k increases, the mostprobable hybridized state will be only a partially hybridized complex(corresponding to a minimum of the free energy as a function of thenumber of bases formed).

FIG. 19 shows cell viability after exposure to UV light. The cellviability was determined with respect to control cells that were nottreated with UV light. 5 levels of light source intensities were usedfrom minimum (1) to maximum (5). Level 1 intensity: 7.01 W; level 2intensity: 9.6 W; level 3 intensity: 13.2 W; level 4 intensity: 18.4 W;level 5 intensity: 23 W. Error bars indicate standard deviation withn=10 measurements.

DETAILED DESCRIPTION

A reconfigurable DNA nano-tweezer is disclosed herein that can beswitched between a closed and open state with a pulse of light. In itsinitial state, the tweezer is held shut using a hairpin with asingle-stranded loop. Also incorporated in the structure is a triggerstrand that is complementary to the single-stranded loop and includesphotocaged residues. Upon illumination with a given wavelength of light,the cages are released and the trigger strand hybridizes to the hairpinloop, opening the tweezer and increasing the distance between its arms.This intramolecular process is roughly 60 times faster than adding anexternal trigger strand, and provides a mechanism for the rapidinterconversion of DNA nanostructures with light.

The DNA nano-tweezer structures comprising photocaged residues asdescribed herein are useful for studying receptor-ligand interactions,releasing cargo for drug delivery, or constructing dynamic materials,such as artificial muscles. For example, photocaged DNA nano-tweezersmay be used to assemble light-activated nano-robots and spring-loadedmechanical assemblies, and to achieve on-demand cargo release from atargeted nano-cage.

As used herein, “photocaged DNA nano-tweezer,” refers to a DNAnano-tweezer which has a trigger strand that includes at least onephotocaged residue, the trigger strand being incorporated into thestructure of the DNA nano-tweezer as opposed to being an externaltrigger strand added separately. Upon exposure to a pulse of light, thephotocages are released from the trigger stand and the trigger strandhybridizes to the single-stranded hairpin loop inducing a change in theDNA tweezer from the closed conformation to the open conformation.

As used herein, “DNA nano-tweezer” refers to a nanoscale structureincluding a hairpin with a single-stranded loop and a first arm and asecond arm linked by a crossover hinge wherein the distance between thetip of the first arm and the tip of the second arm is reversibly orirreversibly controlled by binding and release of a trigger strand tothe single-stranded loop of the hairpin. The trigger strand may beattached to either the first arm or the second arm and typically has afree end unattached to the DNA nano-tweezer. Alternatively, the triggerstrand can be external to the DNA nano-tweezer. It will be readilyunderstood by one of ordinary skill in the art that the flexibility andsize of the DNA nano-tweezer may be manipulated by changing the size andsequences of DNA used in constructing the DNA nano-tweezer. In someembodiments, the first arm and second arm are double-crossover tilearms. In some embodiments, a more ridged multi-helix origami assemblymay be utilized. One embodiment of a DNA nano-tweezer in both the closedand open conformation is depicted in FIG. 1A. Conventional DNAnano-tweezer structures are known in the art. See for example Liu et al.(“A DNA tweezer-actuated enzyme nanoreactor,” Nature Communications,2013, 4:2127) and Zhou et al. (“Reversible regulation of protein bindingaffinity by a DNA machine,” J. Am. Chem. Soc., 2012, 134(3), 1416-1418).

As used herein, “closed conformation” refers to the conformation of theDNA nano-tweezer wherein the hairpin loop is free and unbound by atrigger strand. In the closed conformation, the distance between the tipof the first arm and the tip of the second arm is about 4 nm (e.g., 3,4, 5, or 6 nm). In some embodiments, the distance between the tip of thefirst arm and the tip of the second arm in the closed conformation isbetween about 3 nm and about 18 nm, between 3 nm and 16 nm, between 4 nmand 14 nm, or between 4 nm and about 10 nm. In some embodiments, thedistance between the tip of the first arm and the tip of the second armis less than 18 nm, less than 17 nm, less than 16 nm, less than 15 nm,less than 14 nm, less than 13 nm, less than 12 nm, less than 11 nm, lessthan 10 nm, less than 9 nm, less than 8 nm, less than 7 nm, less than 6nm, less than 5 nm, less than 4 nm, less than 2 nm, or less than 1 nm.

As used herein, “open conformation” refers to the conformation of theDNA nano-tweezer wherein the trigger strand is bound to the hairpinloop. In the open conformation, the distance between the tip of thefirst arm and the tip of the second arm is about 16 nm (e.g., 12 nm, 13nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, or 20 nm). In someembodiments, the distance between the tip of the first arm and the tipof the second arm in the open conformation is between about 12 nm andabout 20 nm, between about 13 nm and about 19 nm, between about 14 nmand about 18 nm, or between about 15 nm and about 17 nm. In someembodiments, the distance between the tip of the first arm and the tipof the second arm in the open conformation is at least 12 nm, at least13 nm, at least 14 nm, at least 15 nm, at least 16 nm, at least 17 nm,at least 18 nm, at least 17 nm, at least 20 nm, at least 30 nm, or atleast 40 nm.

In various embodiments of the DNA nano-tweezers described herein,binding of the trigger loop to the hairpin loop results in an increasein the distance between the tip of the first arm and the tip of thesecond arm. The increase in distance may be an increase of about 8 nm,10 nm, 11 nm, 12 nm, 13 nm, 14 nm, or 16 nm.

As used herein, “trigger strand” refers to a nucleic acidoligonucleotide that is complementary to and binds to the hairpin loopof the DNA nano-tweezer to initiate a conformation change in the DNAnano-tweezer from the closed conformation to the open conformation. Thetrigger strand may be between about 14 bases and about 40 bases (e.g.,15 to 35 bases, 18 to 30 bases, 20 bases to 28 bases) in length. In someembodiments, the trigger strand is about 21 bases in length (e.g., 15bases, 16 bases, 17 bases, 18 bases, 19 bases, 20 bases, 21 bases, 22bases, 23 bases, 24 bases, or 25 bases). In some embodiments, thetrigger strand is a poly-T trigger strand that includes about 13 thymine(T) nucleotides, a poly-G trigger strand that includes about 13 guanine(G) nucleotides, or a poly-C trigger strand that includes about 13cytosine (C) nucleotides. In some embodiments, one or more of theresidues in the trigger strand are photocaged residues. In someembodiments, at least half of the residues in the trigger strand thatare complementary to the hairpin loop are photocaged residues. In someembodiments, the trigger strand includes 5, 6, 7, 8, 9, 10, 11, 12 or 13photocaged residues. In some embodiments, residues in the trigger loopalternate between regular nucleic acids and photocaged residues.

As used herein, “photocaged residues” refers to nucleic acids that havebeen modified with a photo-labile protecting group that is released fromthe nucleic acid upon exposure to a wavelength of light specific to thephoto-labile protecting group. In some embodiments, the photo-labileprotecting group is a nitrobenzyl caging group. In some embodiments thephotocaged residues are modified with 6-nitropiperonyloxymethyl (NPOM)which is released from the residue upon exposure to UV light. In someembodiments, the photocaged resides are modified with a1-(2-nitrophenyl)-1-ethyl (NPE) group which is released from the residueupon exposure to UV light. In some embodiments, the photocaged residuesare modified with a 2-(2-nitrophenyl)ethyl (NPP) group which is releasedfrom the residue upon exposure to UV light. Synthesis and use ofphotocaged residues are known and understood in the art: see for exampleBuff et al. (“Light-activated nucleic acids ‘caged’ at the nucleobases,”Chimia 63, 2009, 261-264), Walbert et al. (“Photolabile protectinggroups for nucleosides: mechanistic studies of the2-(2-nitrophenyl)ethyl group,” Helvetica Chimica Acta, 2001, 84(6):1601-1611), Pirrung et al. (“Photoremoveable protecting groups in DNAsynthesis and microarray fabrication,” Chapter 6 of Dynamic Studies inBiology: Phototriggers, Photoswitches and Caged Biomolecules, 2005),Dieters (“Light activation as a method of regulating and studying geneexpression,” Curr Opin Chem Biol, 2009, 13(5-6):678-686), and Lusic etal. (“A new photocaging group for aromatic N-herterocycles,” Synthesis,2006, 13:2147-2150), and Lusic et al. (“Photochemical DNA activation,”Org. Lett., 2009, 9(10):1903-1906).

In some embodiments, the DNA nano-tweezers may additionally include alocking strand on each of the first and second arms to lock the DNAnano-tweezer in the closed conformation. The locking strands form aduplex that more tightly pulls the tips of the first and second armscloser together. The locking strands form a duplex of about 16 bp (e.g.,12 bp, 13 bp, 14 bp, 15 bp, 16 bp, 17 bp, 18 bp, 19 bp, or 20 bp). Insome embodiments, the locking strand also includes a photocleavablebackbone, which cleaves the backbone of the locking strand when exposedto UV light. In some embodiments, the locking strand includes ano-nitrobenzyl ester photocleavable backbone.

In some embodiments, the DNA nano-tweezers may additionally include abound ligand, enzyme, active agent, antibody, cargo molecule, or othercovalently or noncovalently linked moiety. In some embodiments, each armof the DNA nano-tweezer is bound to a different ligand. In someembodiments, the DNA nano-tweezer is bound to ligands that bind dimericreceptors. In some embodiments, the first arm of the DNA nano-tweezer isbound to a ligand and the second arm is bound to a receptor for saidligand. In some embodiments, the DNA nano-tweezer is bound to one ormore proteins. In some embodiments, the DNA nano-tweezer is bound to apolymer. In some embodiments, the DNA nano-tweezer is bound to a polymerand integrated into a larger material such as a hydrogel matrix.

In some embodiments, the DNA nano-tweezer may be labeled with afluorescent label. In some embodiments, the DNA nano-tweezer may belabeled with two or more fluorescent labels. In some embodiments, theDNA nano-tweezer includes a donor-acceptor pair of fluorescent labels,such as would be useful for fluorescence resonance energy transfer(FRET) experiments.

In some embodiments, the DNA nano-tweezer is part of a nano-robot,nano-assembly, or nano-cage. Suitable DNA nano-cage assemblies have beenprevious described in that art. See for example U.S. Patent PublicationNo. 2018/0016569.

Also described herein are methods for reconfiguration of a DNAnano-tweezer described herein. Methods of reconfiguration of the DNAnano-tweezer include the step of exposing a DNA nano-tweezer describedherein to a pulse of light for a length of time sufficient to releasethe protecting groups of the photocaged residues of the trigger loop,whereby the trigger loop binds to the single-stranded loop of thehairpin and the DNA nano-tweezer is reconfigured. In some embodiments,the pulse of light lasts between about 0.05 seconds and about 10seconds, between about 1 second and about 9 seconds, between about 2seconds and about 8 seconds or between about 1 second and about 6seconds. In some embodiments, the light is UV light. In someembodiments, the light is UV light having a wavelength between about 300nm and about 400 nm, between about 320 nm and about 390 nm, betweenabout 330 nm and about 380 nm, or between about 350 nm and about 375 nm.In some embodiments, the light is UV light at a wavelength of about 365nm.

The present invention has been described in terms of one or morepreferred embodiments, and it should be appreciated that manyequivalents, alternatives, variations, and modifications, aside fromthose expressly stated, are possible and within the scope of theinvention.

Example 1

The embodiment described here demonstrates incorporation of a triggerstrand into a DNA nanostructure from the outset while preventing it frombinding to its target by modifying it with photocleavable protectinggroups, thereby making it possible to actuate that structure with light.Light is an ideal stimulus for this purpose because it is clean, fast,and can be controlled in both space and time with high precision,especially for switching the target in the presence of cells.Furthermore, the high effective local concentration enforced byincorporating the trigger strand into the nanostructure should result inextremely fast kinetics, even at equimolar stoichiometry. Currently, thepredominant mechanism for actuating DNA nanostructures using lightemploys the cis-trans isomerization of azobenzene-modifiedoligonucleotides to change the melting temperature of two complementarystrands. Although this elegant approach is highly reversible, for manyapplications (for example, cargo release in a cell) a single switchingis sufficient, or reversibility may be undesired. Furthermore, someazobenzenes can be reduced and inactivated by endogenous thiols insideof cells. Herein, we present the fast and irreversible switching betweentwo states of a DNA nano-mechanical tweezer by incorporating aphotocaged displacement strand into the structure and uncaging it with abrief pulse of ultraviolet (UV) light. Although a number of examplesexist where toeholds are exposed through photo-cleavage reactions, [14]to our knowledge this is the first demonstration of direct photocagingof the displacement strand itself, providing a broadly applicable newmechanism for rapidly switching DNA nanostructures.

Our photoactivated nanostructure is based on a previously reported DNAtweezer design, which consists of two double-crossover tile arms linkedby a crossover hinge (FIG. 1A). In this previous study, asingle-stranded hairpin loop (red, 5′-TGCGTAAGACCCACAATCGCT-3′, SEQ IDNO:9) serves as an actuation element between two arms. The nano-tweezeropening is driven by addition of an external DNA trigger strand (blue,5′-CGTGTGGTTGAGCGATTGTGGGTCTTACGCA-3′, SEQ ID NO:10) that iscomplementary to the loop. The opening kinetics and yield, however,depend on the trigger strand-to-tweezer molar ratio and diffusion inbulk.

In the photo-tweezer design shown in FIG. 1B, the trigger strand (blue,5′-TGCGXTXTXTXTXTXTXCGCT-3′, SEQ ID NO:11) is incorporated into thestructure by appending it to one of the component strands, and placednext to the regulatory hairpin loop (red, 5′-AGCGAAAAAAAAAAAAACGCA-3′,SEQ ID NO:12). Here, 7 thymidines (blue, X) in the internal triggerstrand are protected with the 6-nitropiperonyloxymethyl (NPOM) caginggroups pioneered by the Deiters group for regulating biologicalprocesses, thereby preventing hybridization between it and the hairpinloop. The photocaged strand was purchased from a commercial vendor,which will allow ready access to this technology. We reasoned that uponbrief exposure to UV light, the NPOM groups would be rapidly cleaved,allowing the internal trigger strand to hybridize with the loop and openthe tweezer.

We first monitored the deprotection process by UV/Vis spectroscopy (FIG.1C). The NPOM group has a signature absorbance at 365 nm, which wasreadily observed for the photocaged trigger strand containing 7 cages(blue). How-ever, after UV illumination for 3 s the peak was reducedalmost to baseline due to removal of cages (purple). We estimate thatmore than 85% of cages were removed by this brief pulse as determinedfrom the UV/Vis spectrum. Control DNA oligonucleotides with the samesequence did not show the 365 nm peak before or after UV irradiation.The deprotection of cage groups was further observed by denaturingpolyacrylamide gel electrophoresis (PAGE) (FIG. 1D). The bandcorresponding to the caged strand (lane 2) quantitatively shifted to afaster migrating band (lane 3) with the same mobility as thenon-protected control strand (lane 1). By native PAGE (FIG. 1E), thephotocaged strand (lane 3) did not bind to its complementary polyAstrand until after deprotection by UV (lane 4), and the duplex has thesame mobility as a control strand-polyA duplex in lane 2.

Next, we assembled tweezers bearing both the hairpin loop and thephotocaged displacement strand, and probed the UV light-induced openingby both atomic force microscopy (AFM) and Förster resonance energytransfer (FRET) with a Cy3-Cy5 donor-acceptor pair (System (i), FIGS.2A, 2C). Before irradiation, the tweezers displayed a distribution ofdistances between the ends of the inner arms (measured by AFM) centeredaround 8 nm. Following UV illumination, however, the tweezers shifted toa more open population centered around 18 nm, closely matching thepredicted model. In several images, the duplex holding the arms apartcan be clearly visualized, further supporting our proposed mechanism. Wenote that the broad distributions of both the closed and open states areexpected due to the relative flexibility of the tweezer; more rigidmulti-helix origami assemblies could be used in the future to narrow thedistance distribution. The opening was further supported by FRET (FIG. 7), with a drop from 22.9% to 15.0% efficiency from the closed to theopen state, which we confirmed was not due to damage of the dyes to theUV illumination (FIGS. 8C-8D). We were unable to correlate the FRETefficiency with precise distances, however, owing to the restrictedrotation of the internally placed donor-acceptor dye pair, whichprecludes the usual approximation of k2=⅔ in the Förster equation forfreely rotating dyes. As a result, FRET could only be used toqualitatively validate the AFM histograms.

We next compared System (i) with the original tweezer actuated by anexternal trigger strand (System (ii)). By both AFM and FRET, we observeda similar shift from the closed to the open states as for thephoto-tweezer. The closed state distance distribution for System (ii)was centered around a slightly smaller distance by both measurements,which we attribute to the polyA hairpin loop in System (i) forcing thetweezer slightly more open due to enhanced base-stacking of the adenineresidues. Interestingly, when we attempted to open a polyA hairpintweezer with an externally added polyT trigger strand, no appreciableopening was observed (FIG. 4B, FIGS. 8A-8D), which is most likely due tothe weaker A-T interactions in this system compared with the optimizedsequence containing several C-G pairs in System (ii). This resulthighlights the potent effect of high local concentration in thephoto-tweezer driving interactions that would not be possible in thebulk solution.

Although the photo-tweezer behaved quite similarly to the original,externally actuated system, the overlapping distance distributions andthe switch from 8 to 18 nm may not be sufficient for some applications.Both Systems (i) and (ii) are partly open even in the closed state (8 nmvs. ca. 4 nm expected from the model), which is most likely due toelectrostatic repulsion between the arms, the flexibility of a tweezerheld together by a single crossover at the hinge, and the presence ofonly 3 base pairs in the stem of the hairpin loop. We thus next asked ifwe could assemble a more compact closed state by extending the two armswith two complementary locking strands that could form a 16-bp duplex(System (iii), FIG. 2B). To render the system photo-responsive, weintroduced an o-nitrobenzyl ester photocleavable backbone modification(orange star), which has a maximum absorbance between 300 and 350 nm,into one of the locking strands. We reasoned that upon UV illumination,the backbone cleavage would release the locker strands simultaneouslywith NPOM cleavage, allowing the tweezer to switch to the open state.Indeed, prior to UV exposure System (iii) showed a peak inter-armdistance around 3-4 nm, but the open tweezer distribution was virtuallyidentical to System (i) at 18 nm. Relative to System (i), the FRETefficiency for System (iii) was significantly enhanced for the closedstate (58.2%), and slightly lower for the open state (10.3%).Furthermore, there is virtually no overlap in the distance histogramsbetween the closed and open states for System (iii), which will beuseful especially for applying forces on biological systems likeproteins that require a clear differentiation of the two states.

As controls, we generated analogues of Systems (i) and (iii), termedSystems (iv) and (v), respectively, but with a polyT hairpin loop thatshould not bind to the internal trigger strand after removing thephotocages. As expected, both of these controls were similar to theirrespective systems in the closed configurations by both AFM and FRET.However, after irradiation, System (iv) showed no change in the distancedistribution, confirming the inability of the trigger strand to bind thehairpin. System (v) showed a broad open configuration after UVirradiation, which was perhaps due to additional electrostatic repulsionintroduced by the duplex locker strands, yet was clearly less open thanSystem (iii).

Having demonstrated that the photocaged displacement strand approach wasefficient and stimulus-responsive, we next turned to characterizing thekinetics of the system. A central hypothesis of our approach is that thehigh local concentration of the photocaged strand should allow for muchmore rapid nanostructure actuation compared with externally addedstrands. To probe this effect, we carried out a series of time-dependentFRET experiments (monitoring Cy5 acceptor emission), triggering tweezeropening with either UV light or increasing concentrations of theexternal trigger strand, with Systems (iii) and (ii), respectively(FIGS. 3A-3B). For System (iii), UV irradiation resulted in a rapid dropin Cy5 emission, reaching a minimum value after only 25 seconds (FIG.3A). The normalized kinetic traces were independent of tweezerconcentration (10 vs. 60 nm), as expected for this effectivelyunimolecular process. To quantify the opening, we fitted the curves toequations (1) and (2), and calculated the time to reach 90% of the openFRET emission, defined as t90 (Table 1). For System (iii), t90=15 s. Bycontrast, 1 equivalent of externally added trigger strand for System(ii) (at a tweezer concentration of 60 nm) showed dramatically slowerkinetics (FIG. 3B), with t90=915 s, corresponding to a roughly 60-foldrate reduction relative to the internally incorporated photocagedstrand. Even at 25 equivalents of trigger strand (1.5 mm concentration)t90=24 s, which was still slower than System (iii).

Taken together, our results highlight the great potential for internalphotocaged displacement strands as a way to switch quickly andirreversibly between two conformational states for a nano-mechanicaldevice. We envision that this approach will be particularly useful forexerting forces on biological systems at the nanoscale in a highlystimulus-responsive manner. For example, functionalizing the tweezerswith ligands that bind dimeric receptors would allow one to rapidlybreak the protein interaction with light and probe biological effects.Toward this end, we used computational simulations to estimate the rangeof forces that can be applied by System (iii) as up to about 46 pN,which is well within the range of many biological sensing events. Theabove experiments used a brief, intense UV pulse that proved harmful tocells (ca. 50% survival). However, by reducing the exposure time andusing a UV source with an emission spectrum more narrowly tailored tothe NPOM absorbance, we were able to improve cell survival to >85%,making our system relevant for biological studies with live cells.Finally, we note that by designing multiple displacement strands, withorthogonal sequences, it should be possible to reconfigure complex DNAnanostructures (for example, 3D origami assemblies), leading tolight-activated nano-robots, spring-loaded mechanical assemblies, oron-demand cargo release from a targeted nano-cage.

Materials

DNA Strands:

Single-stranded oligonucleotides, fluorophore (Cy3/Cy5)-modifiedoligonucleotides and photo-cleavable linker modified oligonucleotideswere purchased from IDT DNA (Integrated DNA Technologies, Inc.).Photocaged oligonucleotides were purchased from Bio-Synthesis, Inc.(Lewisville, Tex.).

Buffers:

Tris base, acetic acid, EDTA, and magnesium acetate were purchased fromSigma Aldrich.

Cell Lines:

KB (ATCC® CCL-17™), 293 [HEK-293] (ATCC® CRL-1573™) and Hep G2 [HEPG2](ATCC® HB-8065™) were purchased from ATCC.

Cell Culture Media and Cell Viability Assay Reagents:

Dulbecco's Modification of Eagle's Medium (DMEM) was purchased fromCorning. Supplemental Fetal Bovine Serum (FBS) was purchased fromAtlanta Biologicals, Inc. Penicillin, Steptomycin and Amphotericin Bantibiotics were purchased from Lonza.

Design, Assembly, and Characterization of DNA Nano-Tweezers

DNA nanostructure design: The detailed sequence designs of the originalDNA Nano-tweezers and photocaged DNA nano-tweezers are shown in FIGS.4A-4D. Tiamat (downloaded from yanlab.asu.edu/Resources.html) was usedfor structure and sequence design. All sequences are written in 5′-3′order.

Sequences of original DNA nano-tweezers: T1: (SEQ ID NO: 13)TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2: (SEQ ID NO: 14)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTAT CTGCTCTTT T2-iCy5:(SEQ ID NO: 15) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAAGCTATCTGCTCTTT T3: (SEQ ID NO: 16)TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAAC TGCTGCC T3-iCy3:(SEQ ID NO: 17) TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T4: (SEQ ID NO: 18)TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5: (SEQ ID NO: 19)TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTT T7:(SEQ ID NO: 21) GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCTGCGTAAGACCCACAATCGCTACTATTCATTAACGTTGGTACGAACGTAACCTGGC AACGGAG T8:(SEQ ID NO: 22) TTTTAGAGCACCGCTCGGTCGTTT T9: (SEQ ID NO: 23)TTTCAGACGACCATCTCCTTT External trigger strand: (SEQ ID NO: 10)CGTGTGGTTGAGCGATTGTGGGTCTTACGCASequences of photocaged DNA nano-tweezers without locking strands T1:(SEQ ID NO: 13) TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2:(SEQ ID NO: 14) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTATCTGCTCTTT T2-iCy5: (SEQ ID NO: 15)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAA GCTATCTGCTCTTT T3:(SEQ ID NO: 16) TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T3-iCy3: (SEQ ID NO: 17)TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATC CTAACTGCTGCC T4:(SEQ ID NO: 18) TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5:(SEQ ID NO: 19) TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-1-Aloop: (SEQ ID NO: 24)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCAGCGAAAAAAAAAAAAACGCAACTATTCATTAACGTTGGTACGAACG T7-2-photocage: (SEQ ID NO: 25)TAACCTGGCAACGGAGTGCGXTXTXTXTXTXTXCGCT T8: (SEQ ID NO: 22)TTTTAGAGCACCGCTCGGTCGTTT T9: (SEQ ID NO: 23) TTTCAGACGACCATCTCCTTTSequences of photocaged DNA nano-tweezers with locking strands T1:(SEQ ID NO: 13) TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2:(SEQ ID NO: 14) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTATCTGCTCTTT T2-iCy5: (SEQ ID NO: 15)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAA GCTATCTGCTCTTT T3:(SEQ ID NO: 16) TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAAC TGCTGCC T3-iCy3: (SEQ ID NO: 17)TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATC CTAACTGCTGCC T4:(SEQ ID NO: 18) TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5:(SEQ ID NO: 19) TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-1-Aloop: (SEQ ID NO: 24)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCAGCGAAAAAAAAAAAAACGCAACTATTCATTAACGTTGGTACGAACG T7-2-photocage: (SEQ ID NO: 25)TAACCTGGCAACGGAGTGCGXTXTXTXTXTXTXCGCT T8-lock-PC: (SEQ ID NO: 26)CATCAGTCGAGCTGC/iSpPC/CAGAGCACCGCTCGGTCGTTT T9-lock: (SEQ ID NO: 27)CAGACGACCATCTGGCAGCTCGACTGATG Sequences of control group (polyA loop andinternal polyA strand) without locking strands T1: (SEQ ID NO: 13)TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2: (SEQ ID NO: 14)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTAT CTGCTCTTT T2-iCy5:(SEQ ID NO: 15) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAAGCTATCTGCTCTTT T3: (SEQ ID NO: 16)TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAAC TGCTGCC T3-iCy3:(SEQ ID NO: 17) TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T4: (SEQ ID NO: 18)TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5: (SEQ ID NO: 19)TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-1-Aloop: (SEQ ID NO: 24)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCAGCGAAAAAAAAAAAAACGCAACTATTCATTAACGTTGGTACGAACG T7-2-polyA: (SEQ ID NO: 28)TAACCTGGCAACGGAGTGCGAAAAAAAAAAAAACGCT T8: (SEQ ID NO: 22)TTTTAGAGCACCGCTCGGTCGTTT T9: (SEQ ID NO: 23) TTTCAGACGACCATCTCCTTTSequences of control group (polyA loop andinternal polyA strand) with locking strands T1: (SEQ ID NO: 13)TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2: (SEQ ID NO: 14)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTAT CTGCTCTTT T2-iCy5:(SEQ ID NO: 15) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAAGCTATCTGCTCTTT T3: (SEQ ID NO: 16)TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAAC TGCTGCC T3-iCy3:(SEQ ID NO: 17) TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T4: (SEQ ID NO: 18)TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5: (SEQ ID NO: 19)TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-1-Aloop: (SEQ ID NO: 24)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCAGCGAAAAAAAAAAAAACGCAACTATTCATTAACGTTGGTACGAACG T7-2-polyA: (SEQ ID NO: 28)TAACCTGGCAACGGAGTGCGAAAAAAAAAAAAACGCT T8-lock-PC: (SEQ ID NO: 26)CATCAGTCGAGCTGC/iSpPC/CAGAGCACCGCTCGGTCGTTT T9-lock: (SEQ ID NO: 27)CAGACGACCATCTGGCAGCTCGACTGATG Sequences of control group (only internalphotocaged strand) without locking strands T1: (SEQ ID NO: 13)TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2: (SEQ ID NO: 14)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTAT CTGCTCTTT T2-iCy5:(SEQ ID NO: 15) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAAGCTATCTGCTCTTT T3: (SEQ ID NO: 16)TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAAC TGCTGCC T3-iCy3:(SEQ ID NO: 17) TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T4: (SEQ ID NO: 18)TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5: (SEQ ID NO: 19)TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-1-no loop: (SEQ ID NO: 29)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCAGCGCGCAACTATTCATTAACGTTGGTACGAACG T7-2-photocage: (SEQ ID NO: 25)TAACCTGGCAACGGAGTGCGXTXTXTXTXTXTXCGCT T8: (SEQ ID NO: 22)TTTTAGAGCACCGCTCGGTCGTTT T9: (SEQ ID NO: 23) TTTCAGACGACCATCTCCTTTSequences of control group (only internalphotocaged strand) with locking strands T1: (SEQ ID NO: 13)TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2: (SEQ ID NO: 14)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTAT CTGCTCTTT T2-iCy5:(SEQ ID NO: 15) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAAGCTATCTGCTCTTT T3: (SEQ ID NO: 16)TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAAC TGCTGCC T3-iCy3:(SEQ ID NO: 17) TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T4: (SEQ ID NO: 18)TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5: (SEQ ID NO: 19)TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-1-no loop: (SEQ ID NO: 29)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCAGCGCGCAACTATTCATTAACGTTGGTACGAACG T7-2-photocage: (SEQ ID NO: 25)TAACCTGGCAACGGAGTGCGXTXTXTXTXTXTXCGCT T8-lock-PC: (SEQ ID NO: 26)CATCAGTCGAGCTGC/iSpPC/CAGAGCACCGCTCGGTCGTTT T9-lock: (SEQ ID NO: 27)CAGACGACCATCTGGCAGCTCGACTGATG Sequences of control group (polyT loop andinternal photocaged strand) with locking strands T1: (SEQ ID NO: 13)TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2: (SEQ ID NO: 14)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTAT CTGCTCTTT T2-iCy5:(SEQ ID NO: 15) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAAGCTATCTGCTCTTT T3: (SEQ ID NO: 16)TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAAC TGCTGCC T3-iCy3:(SEQ ID NO: 17) TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T4: (SEQ ID NO: 18)TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5: (SEQ ID NO: 19)TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-Tloop: (SEQ ID NO: 30)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCAGCGTTTTTTTTTTTTTCGCAACTATTCATTAACGTTGGTACGAACG T7-2-photocage: (SEQ ID NO: 25)TAACCTGGCAACGGAGTGCGXTXTXTXTXTXTXCGCT T8-lock-PC: (SEQ ID NO: 26)ATCAGTCGAGCTGC/iSpPC/CAGAGCACCGCTCGGTCGTTT T9-lock: (SEQ ID NO: 27)CAGACGACCATCTGGCAGCTCGACTGATG Sequences of control group (polyT loop andinternal photocaged strand) without locking strands T1: (SEQ ID NO: 13)TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2: (SEQ ID NO: 14)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTAT CTGCTCTTT T2-iCy5:(SEQ ID NO: 15) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAAGCTATCTGCTCTTT T3: (SEQ ID NO: 16)TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAAC TGCTGCC T3-iCy3:(SEQ ID NO: 17) TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T4: (SEQ ID NO: 18)TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5: (SEQ ID NO: 19)TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-Tloop: (SEQ ID NO: 30)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCAGCGTTTTTTTTTTTTTCGCAACTATTCATTAACGTTGGTACGAACG T7-2-photocage: (SEQ ID NO: 25)TAACCTGGCAACGGAGTGCGXTXTXTXTXTXTXCGCT T8: (SEQ ID NO: 22)TTTTAGAGCACCGCTCGGTCGTTT T9: (SEQ ID NO: 23) TTTCAGACGACCATCTCCTTTSequences of control group (polyA loop andinternal polyT strand) with locking strands T1: (SEQ ID NO: 13)TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2: (SEQ ID NO: 14)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTAT CTGCTCTTT T2-iCy5:(SEQ ID NO: 15) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAAGCTATCTGCTCTTT T3: (SEQ ID NO: 16)TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAAC TGCTGCC T3-iCy3:(SEQ ID NO: 17) TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T4: (SEQ ID NO: 18)TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5: (SEQ ID NO: 19)TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-1-Aloop: (SEQ ID NO: 24)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCAGCGAAAAAAAAAAAAACGCAACTATTCATTAACGTTGGTACGAACG T7-2-polyT: (SEQ ID NO: 31)TAACCTGGCAACGGAGTGCGTTTTTTTTTTTTTCGCT T8-lock-PC: (SEQ ID NO: 26)CATCAGTCGAGCTGC/iSpPC/CAGAGCACCGCTCGGTCGTTT T9-lock: (SEQ ID NO: 27)CAGACGACCATCTGGCAGCTCGACTGATG Sequences of control group (polyA loop andinternal polyT strand) without locking strands T1: (SEQ ID NO: 13)TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2: (SEQ ID NO: 14)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTAT CTGCTCTTT T2-iCy5:(SEQ ID NO: 15) TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGC/iCy5/TAAGCTATCTGCTCTTT T3: (SEQ ID NO: 16)TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAAC TGCTGCC T3-iCy3:(SEQ ID NO: 17) TTTGATGGAACGTTAAT/iCy3/AATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T4: (SEQ ID NO: 18)TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5: (SEQ ID NO: 19)TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-1-Aloop: (SEQ ID NO: 24)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCAGCGAAAAAAAAAAAAACGCAACTATTCATTAACGTTGGTACGAACG T7-2-polyT: (SEQ ID NO: 31)TAACCTGGCAACGGAGTGCGTTTTTTTTTTTTTCGCT T8: (SEQ ID NO: 22)TTTTAGAGCACCGCTCGGTCGTTT T9: (SEQ ID NO: 23) TTTCAGACGACCATCTCCTTTSequences of photocaged tweezers with 4 cages T1: (SEQ ID NO: 13)TTTTCGACCGAGCGGAAATTAGTGATCCGGAACTCGAGCAATGAACCTT TT T2: (SEQ ID NO: 14)TTTCAGCTGGCCGATCTAAGACTGAACTCTCACCGCCGGCATAAGCTAT CTGCTCTTT T3:(SEQ ID NO: 16) TTTGATGGAACGTTAATGAATAGTCTCCGATTGCATCCGAGATCCTAACTGCTGCC T4: (SEQ ID NO: 18)TTTTCGAGAGAAGGCTTGCCAGGTTACGTTCGTACCTCGTCTGTTT T5: (SEQ ID NO: 19)TTTTGGCAGCAGTTACGGCCAGCTGATT T6: (SEQ ID NO: 20)TTTTGGTTCATTGCTGAGTTCAGTCTTAGATGGATCTCGGATGCAATGC CTTCTCTCGTTTTT7-1-loop: (SEQ ID NO: 32)GGTGACGAGTTCCGGATCACTAATTTGATAGCTTATGCCGGCTGCGTAAGACCCACAATCGCTACTATTCATTAACGTTGGTACGAACG T7-2-photocage: (SEQ ID NO: 33)TAACCTGGCAACGGAGAGCGAXTGXGGGXCTXACGCA T8: (SEQ ID NO: 22)TTTTAGAGCACCGCTCGGTCGTTT T9: (SEQ ID NO: 23) TTTCAGACGACCATCTCCTTT

Purification of DNA Oligonucleotides:

Photocaged oligonucleotides were purified by BioSynthesis using RP-HPLC.Fluorophore (Cy3/Cy5)-modified oligonucleotides and photocleavablelinker modified oligonucleotides were purified by IDT using RP-HPLC.Other oligonucleotides were purified in lab using previously describedmethod (Liu et al. 2013).

Estimation of NPOM Groups Removal Based on UV-Vis Spectra Measurement.

UV-Vis spectra of photostrand were taken before and after UVillumination. The percentage of NPOM cage molecules removal wasestimated using the following calculation:

$1 - \frac{A_{after} - A_{baseline}}{A_{before} - A_{baseline}}$Where Abefore, Aafter and Abaseline represents the 365 nm absorbance ofphotocaged strand before or after UV illumination and baseline,respectively.

DNA Nano-Tweezers Assembly:

The DNA strands constituting each DNA structure were combined in anequimolar ratio in 1×TAE-Mg²⁺ buffer (40 mM Tris, 20 mM acetic acid, 2mM EDTA and 12.5 mM magnesium acetate, pH 8.0) to reach a finalconcentration of 0.5 μM per strand.

Discussion on Photocaged Sequence Design:

Our initial photo-tweezer design used a random sequence for the hairpinloop and four photocaged residues in the internal trigger strand (FIG.5A). However, this approach resulted in constitutively open tweezers,even prior to UV illumination (FIG. 5B). We hypothesize that this wasdue to the high effective molarity of the trigger (˜2.7 mM) allowingbinding even in the presence of four “mismatches.” However, with UV-visillumination this strand with four cages (termed S1) did undergocomplete photocleavage (FIG. 5C). We next investigated adding photocagedresidues closer together, including adjacent residues (strand S2), aswell as increasing the number of caged residues and including three in arow (strand S3). However, by UV-Vis we observed a reduction inphotocleavage for S2, and virtually no photocleavage for S3 (FIG. 5C).We hypothesized that adjacent residues might prevent efficientphotocleavage due to competing mechanisms, and so settled on the finaldesign with seven caged residues alternating with non-caged dTs. StrandsS2 and S3 were made in-house on a solid-phase oligonucleotidesynthesizer, using reported protocols for the phosphoramidite synthesis.

Bulk FRET Measurements

The efficiency of energy transfer (E) was determined according to thefollowing equation:

$E = {1 - \frac{I_{DA}}{I_{D}}}$where I_(DA) and I_(D) are, respectively, the fluorescence intensitiesof the FRET donor (Cy3) in the presence and absence of the FRET acceptor(Cy5). All FRET calculations were performed relative to Cy3-only controlsamples that had also been exposed to UV to correct for any damage tothe dyes, but we additionally confirmed that such damage was minimal(FIGS. 8C, 8D).Kinetics Measurements

The kinetics for nano-tweezer opening were determined by measuring thetime-dependent fluorescence change between donor and acceptor dyes usinga Nanolog fluorometer (Horiba Jobin Yvon). To ensure the accuracy of thekinetics experiment, sample injection was performed with a stopped flowaccessory (SFA-20, TgK Scientific) that can mix equal volumes of twosamples and inject the mixtures into a cuvette for fluorescencerecording in about 0.01 s (a nominal dead time <8 ms according tomanufacturer's specifications). In a typical experiment, 60 μL each ofthe tweezer and the trigger strand (final concentrations of 60 nM forthe tweezer and 60·n nM of the trigger strand, where n is the fold oftrigger strand) were used for all kinetic measurements. The parameterssettings for the fluorimeter were as follows: 550 nm excitation, 1 nmexcitation slit, 670 nm emission, 10 nm emission slit. The signal wascollected from 0 to 300 s with 0.5 s integration time and 1 s intervals.Kinetic measurements were repeated 4-6 times for each condition at 20°C. The rate constant of the reaction was obtained by fitting the data asdescribed below.

Kinetic Model.

The opening of the photo-caged tweezer is an intramolecular reaction,and so it was analyzed as a first order reaction. For the reaction shownbelow, the rate constant of the forward reaction is k. As thephoto-uncaging process is irreversible, the backward reaction can beneglected. The initial concentrations of the closed tweezer is (C₀).

-   -   T_(close)        T_(open)        The reaction rate can be described as follows:

$r = {\frac{- {d\lbrack T_{close} \rbrack}}{dt} = {k\lbrack T_{close} \rbrack}}$Integrating the above equation gives the following:

${\ln( \lbrack T_{close} \rbrack )} = {{{- {kt}} + {\ln( C_{0} )} - {kt}} = {\ln( \frac{\lbrack T_{close} \rbrack}{C_{0}} )}}$$\frac{\lbrack T_{close} \rbrack}{C_{0}} = e^{- {kt}}$The next step is to relate the equation above to the experimental datawe collected. For each kinetic curve, at time 0, after time t, and atthe end of the reaction (t goes to ∞), the normalized fluorescenceintensities are I₀, I_(t), and I_(∞), respectively.

$\begin{matrix}{\frac{\lbrack T_{close} \rbrack}{C_{0}} = {e^{- {kt}} = \frac{I_{t} - I_{\infty}}{I_{0} - I_{\infty}}}} & \; \\{{Then},} & \; \\{I_{t} = {I_{\infty} + {( {I_{0} - I_{\infty}} )e^{- {kt}}}}} & (1)\end{matrix}$The above equation is used to fit the normalized kinetic curve with timeby using three parameters:I_(∞), I₀, and k.The opening of the trigger strand-actuated tweezer is an intermolecularreaction, and so it was analyzed as a second order reaction. For thereaction shown below, the rate constants of the forward reaction is k.Once again, the backward reaction can be neglected because the freeenergy of hybridization precludes loss of the displacement strand. Theinitial concentrations of the closed tweezer and the trigger strand isC₀ and n·C₀, respectively, where n is the fold added of the triggerstrand.

-   -   T_(close)+Fuel        T_(open)        The reaction rate can be described as follows:

$r = {\frac{- {d\lbrack T_{close} \rbrack}}{dt} = {{{k\lbrack T_{close} \rbrack}\lbrack{Fuel}\rbrack} = {{k\lbrack T_{close} \rbrack}( {n \cdot \lbrack T_{close} \rbrack} )}}}$When n=1, the initial concentrations of the closed tweezer and thetrigger strand are the same (C₀). The reaction rate can be simplified asfollows:

$r = {\frac{- {d\lbrack T_{close} \rbrack}}{dt} = {{{k\lbrack T_{close} \rbrack}\lbrack{Fuel}\rbrack} = {k\lbrack T_{close} \rbrack}^{2}}}$Next, definite integration can be applied to the above equation toobtain the following:

$\frac{\lbrack T_{close} \rbrack}{C_{0}} = \frac{1}{1 + {ktC}_{0}}$The next step is to relate the equation above to the experimental datawe collected. For each kinetic curve, at time 0, after time t, and atthe end of the reaction (t goes to ∞), the normalized fluorescenceintensities are I₀, I_(t), and I_(∞), respectively.

$\begin{matrix}{\frac{\lbrack T_{close} \rbrack}{C_{0}} = {\frac{1}{1 + {ktC}_{0}} = \frac{I_{t} - I_{\infty}}{I_{0} - I_{\infty}}}} & \; \\{{Then},} & \; \\{I_{t} = {I_{\infty} + \frac{I_{0} - I_{\infty}}{1 + {ktC}_{0}}}} & \;\end{matrix}$The above equation is used to fit the normalized kinetic curve with timeby using three parameters:I_(∞), I₀, and k.When n is not 1, the initial concentrations of the closed tweezer andthe trigger strand are different. Then, definite integration can beapplied to the above equation to obtain the following:

$\mspace{20mu}{{\frac{{- {\ln( \lbrack T_{close} \rbrack )}} + {\ln( C_{0} )}}{( {n - 1} )C_{0}} - \frac{{- {\ln( {{( {n - 1} ) \cdot C_{0}} + \lbrack T_{close} \rbrack} )}} + {\ln( {n \cdot C_{0}} )}}{( {n - 1} )C_{0}}} = {{{{kt}( {n - 1} )}{C_{0} \cdot {kt}}} = {{{\ln( {( {n - 1} ) \cdot {C_{0}\lbrack T_{close} \rbrack}} )} - {\ln( \lbrack T_{close} \rbrack )} - {\ln(n)}} = {\ln( {\frac{( {n - 1} ) \cdot C_{0}}{n \cdot \lbrack T_{close} \rbrack} + \frac{1}{n}} )}}}}$$\mspace{20mu}{\frac{\lbrack T_{close} \rbrack}{C_{0}} = \frac{n - 1}{{n \cdot e^{{({n - 1})}{C_{0} \cdot {kt}}}} - 1}}$The next step is to relate the equation above to the experimental datawe collected. For each kinetic curve, at time 0, after time t, and atthe end of the reaction (t goes to ∞), the normalized fluorescenceintensities are I₀, I_(t), and I_(∞), respectively.

$\begin{matrix}{\frac{\lbrack T_{close} \rbrack}{C_{0}} = {\frac{n - 1}{{n \cdot e^{{({n - 1})}{C_{0} \cdot {kt}}}} - 1} = \frac{I_{t} - I_{\infty}}{I_{0} - I_{\infty}}}} & \; \\{{Then},} & \; \\{I_{t} = {I_{\infty} + {( {I_{0} - I_{\infty}} )\frac{n - 1}{{n \cdot e^{{({n - 1})}{C_{0} \cdot {kt}}}} - 1}}}} & (2)\end{matrix}$The above equation is used to fit the normalized kinetic curve with timeby using three parameters:I_(∞), I₀, and k.

TABLE 1 Calculated time to reach 90% of the open FRET emission forSystem ii. Ration (trigger/ tweezer) 1 2 5 10 20 22 25 t₉₀ (s) 915 25489 38 33 29 24Computational Simulations to Determine Force Exerted by Tweezer

The DNA photo-tweezer introduced in this work can be used for a numberof applications that require applying a rapid force at the nanoscaleupon illumination. For example, the tweezer could be used as a nanoscalemolecular machine—pulling apart two bound proteins, a ligand from itsreceptor, or applying a force to two different points on a proteinsurface—by attaching the relevant components to the ends of the tweezerarms. Thus, we were interested to determine what force this structurecould apply, and over what distance.

The system uses the chemical energy—provided by the uncaged strandbinding to the complementary hairpin loop—to pull apart the armsconnected by a newly hybridized duplex. If duplex DNA behaved as a rigidrod, we could assume that most of the binding free energy could be usedto pull apart the components bound to the tweezer arms. However, onshort length scales DNA has been shown to be much more bendable thanimplied by a worm-like-chain (WLC) model with persistence length of 50nm, which is typically used to describe the mechanical behavior of dsDNA(see for instance experimental results in Fields et al. 2013 and areview in Vologodskii et al. 2013). It turns out that the DNA duplex can“kink” (as illustrated in FIG. 14 ), thereby bending sharply at a muchlower energy cost than predicted by the WLC model.

To study the range of forces that can be achieved by the tweezer system,we studied the hybridization process of the complementary strands viacomputer simulations using oxDNA: a coarse-grained model parametrized tocapture basic structural, mechanical, and thermodynamic properties ofDNA. It has an accurate representation of properties of bothsingle-stranded and double-stranded DNA, and has been applied to avariety of DNA nano-technological systems (for example, see Doye et al.2013 for an overview). In particular, it has been shown to reproduce DNAbending and kinking behavior as observed experimentally (Harrison et al.2015, Harrison et al. 2015).

In oxDNA, we simulated the hybridization of two complementary strands(the same sequences used in the photo-tweezer). We introduced a harmonicpotential between the first and last bases of one of these strands,defined as:

$\begin{matrix}{{V(r)} = {\frac{k}{2}( {r - r_{0}} )^{2}}} & (3)\end{matrix}$where r is the distance between the two bases, and r₀=1 nm correspondsto a typical distance between two hydrogen-bonded bases in the model.The typical configuration of the system when the two complementarystrands are not bound is illustrated in FIG. 14A. We ran the Monte Carlosimulations to sample the hybridization of the two strands (see forexample Sulc et al. 2012 for a detailed description of the hybridizationsampling simulation), and measured the values of the harmonic springpotential (given by Eq. (3)) during the various stages of thehybridization, from which the corresponding force can be extracted(shown in FIG. 14 ). We tested a range of stiffness values k for thespring potential. When the spring energy becomes roughly comparable to,or larger, than twice the typical stacking energy between adjacent basepairs—roughly 95 pN/nm in the model—the duplex prefers to kink ratherthan extend the spring potential. During kinking, the spring-connectedbases stay in proximity, while the duplex is bent sharply, either due tothe breaking of a base pair or by breaking two stacking interactions (asshown in FIG. 14 ). The maximum values of the force applied by thespring potential (Eq. (3)) observed in the hybridized state in thesampling simulation was approximately 46 pN, as shown in FIG. 17 . For agiven energy value of V(r), the larger the stiffness k, the smaller thedistance r between the spring-joined bases. This limits the maximumdistance at which the tweezer would be able to pull apart for instancetwo interacting proteins. Note however that as the stiffness kincreases, the system will prefer to form only partially formed ofkinked duplex (illustrated by the free energy profile in FIG. 19 ), andhence we would expect the most likely state of system to be when thetweezer opens partially or does not open at all.

We note that these simulations only considered forces applied at the endbases of the hairpin. Using the full tweezer would involve applyingforces at the end of the arms, potentially kinking the duplexes thatcomprise them. Nevertheless, our results provide an approximate value ofthe forces that can be exerted by opening bases of a hairpin viahybridization. The total force exerted by the tweezer could be furtherenhanced by combining multiple hairpin elements into a single structure,like a large DNA origami caliper (Funke et al., 2016).

Cell Viability Assay

Cells were pre-seeded in black 96-well plates with clear bottom at 3×10⁴cells per well and allowed to grow for 24 hours. Sample rows were thenexposed ˜2 cm under Dymax BlueWave® 200 version 3.0 UV curing spot lamp(newer model, 5 levels of light source intensities from minimum tomaximum) while control rows were blocked from UV. Treated cells wereallowed to grow another 24 hours before the MTT assay. On the next day,cells were washed with PBS buffer twice and 100 μL MTT reagent (5 mg/mL)was added and cells were incubated for 4 hours. After 4 hours, the MTTreagent was removed and the sample was dissolved in 150 μL of DMSO. Theabsorbance was measured at 550 nm. The relative cell viability wasdetermined with respect to the control cell incubated with DMEM.

We claim:
 1. A DNA nano-tweezer comprising: a DNA hairpin with asingle-stranded loop, a first arm, and a second arm, wherein thedistance between the first arm and the second arm is between about 3 nmand about 18 nm; and a DNA trigger strand complementary to thesingle-stranded loop and comprising at least one photocaged residue witha protecting group.
 2. The DNA nano-tweezer of claim 1, wherein thesingle-stranded loop is a poly-A loop the trigger strand is a poly-Tstrand.
 3. The DNA nano-tweezer of claim 1, wherein the single-strandedloop is a poly-C loop and the trigger strand is a poly-G loop.
 4. TheDNA nano-tweezer of claim 1, wherein the single-stranded loop and thetrigger strand are selected from the group consisting of a poly-A loop,a poly-T loop, a poly-G loop, and a poly-C loop.
 5. The DNA nano-tweezerof claim 1, wherein the protecting group is a 6-nitropiperonyloxymethylprotecting group.
 6. The DNA nano-tweezer of claim 1, wherein the DNAnano-tweezer additionally comprises a locking strand.
 7. The DNAnano-tweezer of claim 1, wherein the locking strand comprises ano-nitrobenzyl ester photocleavable backbone.
 8. The DNA nano-tweezer ofclaim 1 additionally comprising at least one fluorescent label.
 9. TheDNA nano-tweezer of claim 1 additionally comprising a ligand.
 10. TheDNA nano-tweezer of claim 1, wherein the distance between the first armand the second arm is between about 4 nm and about 16 nm.
 11. A methodof inducing a conformational change in nanostructured DNA, the methodcomprising the step of exposing the DNA nano-tweezer of claim 1 to apulse of light, whereby the DNA nano-tweezer undergoes a conformationalchange from a closed conformation to an open conformation.
 12. Themethod of claim 11, wherein the protecting group is a6-nitropiperonyloxymethyl protecting group and the light is UV light.13. The method of claim 11, wherein the light has a wavelength betweenabout 300 nm and about 400 nm.
 14. The method of claim 11, wherein thepulse of light is between about 1 second and about 10 seconds.